1. Field of the Invention
The present invention relates to a liquid crystal display used in a display section of an electronic apparatus.
2. Description of the Related Art
A liquid crystal display has a pair of substrates provided opposite to each other and a liquid crystal layer sandwiched between the substrates. In general, transparent electrodes are formed on each of surfaces of the pair of substrate opposite to each other. The liquid crystal display is enabled for display by applying a voltage between the transparent electrodes to drive the liquid crystal and to thereby control the transmittance of light at each pixel.
Recently, there are increasing demands and diversifying requirements for liquid crystal displays. In particular, improvement in viewing angle characteristics and display quality is strongly demanded. Multi-domain vertical alignment (MVA) type liquid crystal displays are regarded promising as a technique for achieving improved viewing angle characteristics and display quality.
In a general active matrix type liquid crystal display, the cell thickness between the substrates is controlled by spherical spacers made of plastic. Spherical spacers are dispersed on one of the substrates at a spacer dispersing step before the substrates are attached. Thereafter, the substrates are attached, and a liquid crystal is injected. Further, pressurization is carried out such that the cell thickness will be maintained at a value close to the diameter of the spherical spacers. However, a liquid crystal display utilizing spherical spacers has a problem in that it is likely to have variation of the cell thickness attributable to variation of the dispersion density of the spherical spacers and leakage of light attributable to damage on alignment films that can occur when the spherical spacers move.
Recently, as a technique for simplifying manufacturing processes through a reduction of liquid crystal injection time, the one drop filling (ODF) method has been put in use, in which two substrates are attached after dropping a liquid crystal on one of the substrates. When a liquid crystal is injected using the ODF method, it is difficult to distribute spherical spacers uniformly within the plane of a panel because dispersed spherical spacers move when a liquid crystal is dropped. For this reason, spherical spacers cannot be used in a liquid crystal display fabricated using the ODF method.
Under such circumstances, pillar spacers are used in liquid crystal displays fabricated using the ODF method in particular, the pillar spacers being securely formed on one of substrates using a photolithographic process and contacting the other substrate to maintain a cell thickness after the substrates are attached. FIG. 9 shows a configuration of an MVA type liquid crystal display according to the related art having pillar spacers, and FIG. 10 shows a sectional configuration taken along the line X-X in FIG. 9. As shown in FIGS. 9 and 10, the liquid crystal display has a thin film transistor (TFT) substrate 102 and an opposite substrate 104 which are provided opposite to each other and a liquid crystal 106 sealed between the substrates 102 and 104. The TFT substrate 102 has a plurality of gate bus lines 112 extending in the horizontal direction in FIG. 9 on a glass substrate 110. An insulation film 130 is formed on the gate bus lines 112. A plurality of drain bus lines 114 extending in the vertical direction in FIG. 9 are formed such that they intersect the gate bus lines 112 with the insulation film 130 interposed between them. An insulation film 132 is formed on the drain bus lines 114.
A TFT 120 is formed in the vicinity of each of intersections between the gate bus lines 112 and the drain bus lines 114. Transparent pixel electrodes 116 are formed on the insulation film 132 in pixel regions which are surrounded by the gate bus lines 112 and the drain bus lines 114. Storage capacitor bus lines 118 extending in parallel with the gate bus lines 112 are formed such that they traverse the respective pixel regions substantially in the middle thereof. A storage capacitor electrode (intermediate electrode) 119 is formed in each pixel region above the storage capacitor bus line 118 with the insulation film 130 interposed between them.
The opposite substrate 104 has a shielding film (BM film) 148 (not shown in FIG. 9) provided on a glass substrate 111 for shielding light-shield regions between adjoining pixel regions and regions (light-shield portions) above the storage capacitor bus lines 118 (storage capacitor electrodes 119) in the pixel regions. Color filter (CF) resin layers 140 are formed in the pixel regions on the glass substrate 111. A common electrode 142 is formed throughout the substrate over the CF resin layers 140. Linear protrusions 144 extending obliquely relative to edges of the pixel regions are formed on the common electrode 142 as alignment regulating structures for regulating the alignment of the liquid crystal 106. Pillar spacers 150 are formed in the light-shield regions on the common electrode 142, one spacer 150 being provided for a few pixels or several tens pixels. The pillar spacers 150 are provided in positions opposite to the intersections between the gate bus lines 112 and the drain bus lines 114. Pillar spacers 151 are formed in the light-shield portions in the pixel regions, one spacer 151 being provided for a few pixels or several tens pixels. The pillar spacers 151 are provided in positions opposite to the storage capacitor electrodes 119.
FIG. 11 shows another configuration of an MVA type liquid crystal display according to the related art. As shown in FIG. 11, a pixel electrode 116 formed in a pixel region has a plurality of electrode units 116a having a comb-tooth-shaped peripheral section for regulating the alignment of a liquid crystal 106 and connection electrodes 116b for electrically connecting the electrode units 116a. Pillar spacers 151 are formed in light-shield portions in pixel regions in a disposition density of one pixel per a few pixels or several tens pixels. The pillar spacers 151 are provided in positions opposite to the storage capacitor bus lines 118. Point-like protrusions 145 which are alignment regulating structures are formed on an opposite substrate 104 in positions corresponding to the centers of some of the electrode units 116a. 
Leakage of light and the like can occur around the pillar spacers 150 and 151 because abnormalities can occur in the alignment of the liquid crystal 106 in such regions in general. Therefore, the pillar spacers 150 and 151 are provided in light-shield regions or the light-shield portion in the pixel regions such that display failures attributable to leakage of light will not be visually perceived.
The pillar spacers 150 and 151 are provided in a predetermined pattern in the plane of a substrate. A hard liquid crystal display panel having high anti-pressure characteristics can be obtained by increasing the area or number of contacts between the pillar spacers 150 and 151 and the substrate to increase the disposition density of the pillar spacers 150 and 151.
An effective way to provide a liquid crystal display with high luminance and less power consumption is to improve utilization of light by increasing the aperture ratio of pixels. In order to improve the aperture ratio of pixels, it is necessary to increase the area of apertures at pixel regions relatively by reducing the area of light-shield regions and light-shield portions of the pixel regions. However, a reduction in the area of the light-shield regions and the light-shield portions of the pixel regions puts a limitation on the size and position of the pillar spacers 150 and 151. Thus, it is difficult to increase the disposition density of the pillar spacers 150 and 151 by increasing the number of the pillar spacers 150 and 151. A problem therefore arises in that it is difficult to provide a liquid crystal display having a high aperture ratio with high anti-pressure characteristics.
In a liquid crystal display fabricated using the ODF method, panel defects associated with liquid crystal injection such as bubbles and variation of the picture frame can occur when there is only a slight change in the amount of liquid crystal dispensed. The generation of such panel defects is attributable to variation of the height of the pillar spacers 150 and 151, thermal contraction of the liquid crystal 106, and the characteristics of compressive displacement of the pillar spacers 150 and 151. In order to increase the margin of the amount of liquid crystal dispensed, it is basically required to use a flexible liquid crystal display panel whose cell thickness can flexibly follow a change in the amount of liquid crystal dispensed in a region under a light load. A flexible liquid crystal display panel can be provided by disposing the pillar spacers 150 and 151 in a low disposition density. However, a simple reduction of the disposition density of the pillar spacers 150 and 151 reduces the anti-pressure characteristics of the liquid crystal display panel, and variation of the cell thickness can be more easily caused by local pressurization from the outside such as a press on the display surface. As thus described, in a liquid crystal display fabricated using the ODF method, the margin of the amount of liquid crystal dispensed and the anti-pressure characteristics of the display are a trade-off in general.
As a technique which makes it possible to achieve both of a wide margin of the amount of liquid crystal dispensed and high anti-pressure characteristics, there is a liquid crystal display in which pillar spacers having a great height are provided in a low disposition density and pillar spacers having a smaller height are provided in a higher disposal density (see Patent Documents 1 (JP-A-2001-201750) and 2 (JP-A-2003-156750)). In this liquid crystal display, the cell thickness is normally maintained by the pillar spacers having a greater height disposed in a lower density, and the cell thickness is maintained by the pillar spacers having a smaller height disposed in a higher density when a pressure is applied from the outside. Patent Document 1 discloses a method in which plural types of pillar spacers having different heights are formed on the same substrate. Patent Document 2 discloses a method in which pillar spacers of the same height are disposed in different positions with respect to a pixel and in which the pillar spacers are substantially formed as spacers having different heights utilizing steps formed by the thickness of metal wirings on a TFT substrate provided opposite to the spacers.
However, in a liquid crystal display having a high aperture ratio, since there are limitations on the size and position of pillar spacers as already described, it is difficult to provide pillar spacers having different heights in desired disposal densities as described above. Therefore, a liquid crystal display having a high aperture ratio fabricated using the ODF method has a problem in that it is difficult to achieve a wide margin of the amount of liquid crystal dispensed and high anti-pressure characteristics.